Plural resonator cavities tuned to integrally related frequencies



1964 TADAKUNI FUJIl 3,155,858

PLURAL RESONATOR CAVITIES TUNED TO INTEGRALLY RELATED FREQUENCIES Filed Sept. 50, 1960 2nd Gap ('7) 2O Cavity output Resonator Drift Space First P 9 i (l4)? Cavity Resonator INVENTOR. T. Fujii ATTORNEYS United States Patent "ice 3,155,868 PLURAL RESONATOR CAVITIES TUNED T9 INTEGRALLY RELATED FREQUENCIES Tadakuni Fujii, Tokyo, Eapan, assignor to Nippon Electric Company Limited, Tokyo, Japan Filed Sept. 30, 1960, Ser. No. 59,606 Claims priority, application Japan, Oct. 14, 1959, 34/352,529 7 Claims. (Cl. 315-543) This invention relates generally to velocity modulated high-frequency microwave tubes. More particularly, the invention relates to new features of velocity modulated high-frequency microwave signal waves in the frequency range of 100 kilocycles per second and higher, of the type described in chapter 19 of F. E. Termans book, Electronic and Radio Engineering, fourth edition, published 1955, and the publications referred to in the margins thereof. The high-frequency tubes of the present invention embody or may embody all operating elements, methods and features of such tubes as heretofore known and as described in the aforesaid publication, except for the modifications and novel features of the present invention as hereinafter described.

Conventional velocity modulated high-frequency microwave tubes, which include klystrons, magnetrons and traveling-Wave tubes, when designed for smaller wavelength operation, as in the millimeter range of 1 to 10 mm., are decreased in their dimensions, generally in proportion to the wave length. As a result, their physical dimensions become impracticably small. Also, as the efiiciency or Q of cavity resonators decrease in proportion to the square root of the wave length, the resultant higher frequency losses have, as a practical matter, limited their application to above about 3 mm. signal waves. The precision required for their fabrication would render their cost prohibitive below this wave limit.

It is an object of the present invention to provide novel construction and operation means for velocity modulated high-frequency microwave tubes, which overcome the foregoing limitations.

Another object of the invention is to provide velocity modulated tubes of the foregoing types, which are efficient, practical and eliminate critical operating features, and which may be readily fabricated for operation in the wave range below about 3 mm. wave-length.

These and further objects of the invention will be understood from the following description of exemplifications thereof, in connection with the accompanying drawing figure, in which FIG. 1 is a longitudinal, cross-sectional view of a klystron-type tube exemplifying the invention tube.

In the klystron tube shown, a heater 1t and electron gun 11 form the electron beam 12 extending from gun 11 along gun and tube axis 112-1 through the tube to the collector 18 held at high positive potential 22. The first cavity resonator 13 exerts the first velocity modulation on electron beam 12 across its cavity gap 14. The efiective dimension g of gap 14 presented to the beam 12 is significantly different (in accordance with the invention hereof) as compared to the corresponding gap dimensions of prior-art tubes of this type. Such gap principles and the operation of the invention will be set forth in detail hereinafter.

Upon its first velocity modulation at cavity gap 14, the electron beam 12 travels through the conventional drift region 15 and on to the second cavity gap 17, of the output cavity resonator 16. The second cavity gap 17 has physical dimensions which are related to the first cavity gap 14 and the desired output frequency in a unique manner, as will be explained hereinafter. The

Patented Nov. 3, 1964 output cavity resonator 16 is constructed to be resonant, as a cavity, at the desired output frequency of the tube, e.g., in the desired 1-10 mm. wave range. The beam termination 12-2 is dissipated in the collector 18. The output is delivered to an external lead (not shown) through wave guide 21 coupled to output cavity 16 across a slit 19 therein. A wave guide window 20 is provided at flange 22 formed at the end section of wave guide 21.

The velocity-modulated tube, in accordance with the present invention, has the transit angle of the electrons of beam 12 passing through the first gap 14 selected to be at 21r or an integral multiple of 211-. This is in sharp contrast as compared with prior-art conventional velocitymodulated tubes. In prior-art klystrons, for example, the transit time for electrons passing across such gap has always been kept below one quarter of a cycle or 1r/2. With such first gap of the invention, a negative conductance is produced across gap 14 as the result of the.

passage of said electron beam 12. Provided that the absolute value of the aforementioned negative conductance produced across gap 14 by means of the electron beam 12 is made no less than the equivalent shunt conductance produced across the same gap 14 by means of the circuit losses, the composite conductance across gap 14 becomes negative. A high-frequency oscillation at a frequency which is approximately equal to the resonant frequency of the cavity 13 is thereby started. A microwave oscillator operating on such principle is sometimes referred to as a monotron. The first gap 14 has a physical longitudinal length g, which for the rated collector voltage 22 and beam 12 construction, provides the herein-stated Zn (or multiples) transit time.

The present invention using the monotron operation on the electron beam 12 velocity-modulated at gap 14 by the monotron oscillation action (by the n(21r) gap g) has rich higher harmonic components. These harmonic components are utilized to excite the second resonator 16 which is resonant to one of the produced harmonic frequencies. The starting current of a monotron oscillator is, generally speaking, considerably larger than that for a conventional klystron. Once the oscillation starts in the cavity 13, the power available from the monotron action is comparativly high. Thus the electron beam 12 emerging from the gap 14 can be tightly velocity-modulated as a resultant beam rich in harmonic components.

In the course of passing through the drift space 15, density or current modulation components are produced within the electron beam 12 in exactly the same manner as in the case of a conventional multi-cavity klystron. Thus the second cavity resonator 16 at gap 17 is excited by one of the aforementioned harmonic components, so that the required high-frequency output is produced therein and passed as the output signal to the wave guide 21.

The start of the monotron oscillation, per se, is difficult for frequencies in the range below one centimeter wave length. However, the mono-tron oscillations can be readily started and sustained at a centimeter wave length. Furthermore, since the desired high-frequency (below 10 mm. wave length) is not delivered to the external load from the first cavity resonator 13 of the present invention, the equivalent shunt conductance across gap 14 has losses that are kept to a minimum, as they are at least one centimeter long. The monotron oscillations of the tube of the present invention are at a frequency which can be most easily started and sustained, e.g., at the order of 1 cm. or above. The higher required millimeter-wave signal is obtained at gap 17 of output resonator cavity 16, as a powerful millimeter-wave signal obtained without the diificulties encountered with prior-art tubes of the this type.

The second gap 17 of the output resonator is designed in the conventional manner, to have the electron transit time thereacross below 90. The resonant frequency of the output cavity being of the order of the desired output frequency for the tube, extracts the energy from the density or current modulations on the beam 12 passing across gap 17. The first gap 14, however, is constructed to have a 21r or multiple thereof electron transit time relative to its associated fundamental resonant frequency of the first resonant cavity 13. The self-sustaining oscillations at cavity 13 resulting from such condition, provides the harmonic modulations on beam 12 to excite the secondcavity 16 resonant at a multiple higher frequency with respect to the resonant frequency of the first cavity 13.

While the principles of this invention have been described in connection with a high-frequency (millimeter wave) generator or oscillator, other uses, applications and modifications thereof may be made. it is accordingly desired that in construing the breadth of the appended claims, they shall not be limited to the specific exemplifications of the invention described above.

I claim:

1. A velocity-modulated millimeter-Wave tube incorporating first and second cavity resonators with a drift space region therebetween, each resonator having a velocity modulation gap means for passing an electron beam through said resonators and across a respective velocity modulation gap means of each resonator, the electron transit angle of the first resonator gap means being proportioned to correspond substantially to an integral multiple of 21r of the electrons passing therethrough, and the second resonator gap means having a smaller electron transit angle of no greater than 1r/2 for tight coupling of said electron beam and said second resonator; said second cavity resonator being tuned to extract energ from said velocity modulated electron beam at a frequency which is a harmonic of the modulation frequency of said first cavity resonator.

2.- In 'a velocity-modulated high-frequency microwave tube as.;claimed in claim 1, the resonant frequency of said first resonator being approximately an integral submultiple of that of said second resonator.

3. In a velocity-modulated high-frequency microwave tube as claimed in claim 2, said first resonator being operable as a self-oscillator at its fundamental integral subrnultiple frequency, and said second resonator being proportioned to extract power from an harmonic of the density-modulated electron beam passing thercthrough with the frequency of said harmonic being substantially equal to the second-resonator resonant frequency.

4. In a velocity-modulated high-frequency microwave tube as claimed in claim 1, said first resonator being operable as a self-oscillator at its fundamental integral submultiple frequency, and said second resonator being proportioned to extract power from an harmonic of the density-modulated electron beam passing therethrough with the frequency of said harmonic being substantially equal to the second-resonator resonant frequency, and an output Wave guide coupled to said secondrcsonator.

5. In a vclocity-modulated high-frequency microwave tube as claimed in claim 1, the resonant frequency of said first resonator being of the order of one centimeter in wave length.

6. In a velocity-modulated high-frequency microwave tube as claimed in claim 3, the resonant frequency of said first resonator being of the order of one centimeter in wave length and that of the second resonator being less than 10 millimeters.

7. In a velocity-modulated high-frequency microwave tube as claimed in claim 1, the resonant frequency of said first resonator being of the order of one centimeter in wave length and that of the second resonator being less than 10 millimeters.

References Cited in the file of this patent UNITED STATES PATENTS 2,424,959 Alford Aug. 5, 1947 2,469,843 Pierce May 10, 1949 2,565,708 Warnecke et a1. Aug. 28, 1951 2,579,480 Feenberg Dec. 25, 1951 3,012,170 Heil Dec. 5, 1961 FOREIGN PATENTS 692,473 Great Britain June 10, 1953 

1. A VELOCITY-MODULATED MILLIMETER-WAVE TUBE INCORPORATING FIRST AND SECOND CAVITY RESONATORS WITH A DRIFT SPACE REGION THEREBETWEEN, EACH RESONATOR HAVING A VELOCITY MODULATION GAP MEANS FOR PASSING AN ELECTRON BEAM THROUGH SAID RESONATORS AND ACROSS A RESPECTIVE VELOCITY MODULATION GAP MEANS OF EACH RESONATOR, THE ELECTRON TRANSIT ANGLE OF THE FIRST RESONATOR GAP MEANS BEING PROPORTIONED TO CORRESPOND SUBSTANTIALLY TO AN INTEGRAL MULTIPLE OF 2$ OF THE ELECTRONS PASSING THERETHROUGH, AND THE SECOND RESONATOR GAP MEANS HAVING A SMALLER ELECTRON TRANSIT ANGLE OF NO GREATER THAN $/2 FOR TIGHT COUPLING OF SAID ELECTRON BEAM AND SAID SECOND RESONATOR; SAID SECOND CAVITY RESONATOR BEING TUNED TO EXTRACT ENERGY FROM SAID VELOCITY MODULATED ELECTRON BEAM AT A FRE- 